Attention to energy conservation and resource conservation has been increasingly enhanced recently from the viewpoint of global environmental protection. For example, reduction of power consumption is strongly demanded than ever before in a hermetic compressor employed in a freezing system, such as a refrigerator-freezer for household use. Methods of reducing power consumption by increasing efficiency of the hermetic compressor are described next. One is to reduce a sliding loss at sliding portions, typically a clearance between a piston and cylinder, and a clearance between a main shaft and bearing. Another method is to reduce a loss of refrigerant leakage due to leakage of high-pressure refrigerant from a compression chamber to other part via the clearance between the piston and cylinder during compression of the refrigerant.
A method of reducing sliding loss between the piston and cylinder and also reducing a loss of refrigerant leakage via the clearance between the piston and cylinder is disclosed. In this method, a cylinder is a conic trapezoid whose inner diameter increases from a top dead point to bottom dead point. (For example, refer to PTL1).
The above conventional hermetic compressor is described below with reference to drawings.
FIG. 17 is a vertical sectional view of the conventional hermetic compressor described in PTL1. FIG. 18 is a sectional view of a key part around the piston of the conventional hermetic compressor.
Electric-driving element 5 is provided at a bottom part inside closed vessel 1. Compressing element 6 is provided at an upper part inside closed vessel 1. Compressing element 6 is driven via main shaft 9. Rotor 4 of electric-driving element 5 is directly connected to main shaft 9. Compressing element 6 converts the rotation of eccentric shaft 10 to the reciprocating movement of piston 19 via connecting device 20. As a result, piston 19 reciprocates inside cylinder 16. Eccentric shaft 10 is provided on main shaft 9.
Lubricating oil 7 is stored at the bottom of closed vessel 1. This lubricating oil 7 is stored for lubricating a sliding portion, i.e., main shaft 9 and main bearing 18 of compressing element 6. Centrifugal pump 11 provided at a bottom end of main shaft 9 pumps up lubricating oil 7, and supplies it first to the sliding portion of main bearing 18, and then to eccentric shaft 10 at the upper part via viscous pump 12 provided on main shaft 9.
Lubricating oil 7 supplied to eccentric shaft 10 is led to the outer diameter of eccentric shaft 10 so as to lubricate the sliding portion of connecting device 20, and then is dispersed around from end. A part of this dispersed lubricating oil 7 lubricates the sliding portion between an outer circumference of piston 19 in reciprocating movement and an inner circumference of cylinder 16, and also provides an oil seal to a clearance of this sliding portion.
Cylinder 16 is conic trapezoid whose inner diameter increases from dimension Dt to dimension Db from the top dead point to the bottom dead point. Piston 19 is a cylindrical shape, whose outer diameter is the same over the entire length.
In these shapes, a pressure inside compression chamber 17 is not so much increased to the midway of a compression stroke while piston 19 moves from the bottom dead point to the top dead point where refrigerant gas (not illustrated) is compressed. Therefore, the oil-seal effect by lubricating oil 7 scarcely leaks refrigerant even if clearance Cb between piston 19 and cylinder 16 is relatively large. In addition, relatively large clearance Cb generates only a small sliding resistance of piston 19.
Then, when the compression stroke further advances to increase the refrigerant gas pressure inside compression chamber 17 and piston 19 reaches near the top dead point, the pressure inside compression chamber 17 increases up to a predetermined discharge pressure and becomes high temperature and high pressure. This degrades viscosity of lubricating oil 7, and establishes a condition that may allow leakage of refrigerant. However, since clearance Ct between piston 19 and cylinder 16 becomes small at the side of top dead point, the oil-seal effect of lubricating oil 7 works, and reduces leakage of refrigerant. Accordingly, high compression efficiency can be maintained.
Another example of the prior art is provided with a lubrication groove on an outer circumference of the piston so as to encourage feeding of lubricating oil between the piston and cylinder. This increases the oil-seal effect in the clearance between the piston and cylinder. Accordingly, leakage of compressed refrigerant gas between the piston and cylinder via the clearance is reduced, increasing the efficiency of hermetic compressor. (For example, refer to PTL2.)
The above conventional hermetic compressor is described below with reference to drawings.
FIG. 19 is a vertical sectional view of the conventional hermetic compressor disclosed in PTL2. FIG. 20 is the hermetic compressor seen from the direction of arrow A in FIG. 19. FIG. 21 is a sectional view of a key part around the piston of the conventional hermetic compressor disclosed in PTL2.
In FIGS. 19 to 21, electric-driving element 35 and compressing element 36 are disposed in closed-vessel space 32 inside closed vessel 31. Lubricating oil 37 is stored at a bottom part of closed vessel 31. Electric-driving element 35 includes stator 33 and rotor 34 with built-in permanent magnet (not illustrated). Compressing element 36 is driven by electric-driving element 35.
Shaft 38 includes main shaft 39 and eccentric shaft 40. Oiling device 38a includes centrifugal pump 41, vertical hole 43, and horizontal hole 44. Rotor 34 is press-fitted into main shaft 39. Eccentric shaft 40 is formed eccentric to main shaft 39. Oiling device 38a is formed on shaft 38. One end of centrifugal pump 41 is open submerged in lubricating oil 37, and the other end is connected to viscous pump 42. Vertical hole 43 and horizontal hole 44 are provided at opposite sides relative to viscous pump 42, and are open to closed-vessel space 32.
Block 45 includes cylinder 46 and main bearing 48. Piston 49 is reciprocatably inserted to cylinder 46. There are two ring-like oil grooves 51 on the outer circumference of piston 49. Cylinder 46 forms substantially cylindrical compression chamber 47. Main bearing 48 supports main shaft 39. Connecting device 50 couples piston 49 and eccentric shaft 40.
Oil groove 51 is located on an inner circumference of cylinder 46 at a top dead point (top end face 49a of piston is at arrow B), and is connected to closed-vessel space 32 via notched portion 52 at a bottom dead point (top end face 49a of piston is at arrow C).
The operation of the hermetic compressor as configured above is described next.
Rotor 34 of electric-driving element 35 rotates shaft 38, and the rotation of eccentric shaft 40 is transmitted to piston 49 via connecting device 50. This makes piston 49 reciprocate in compression chamber 47. Refrigerant gas from a cooling system (not illustrated) is sucked into compression chamber 47, compressed, and then discharged to the cooling system again. This compression movement is repeated.
While the hermetic compressor is operated, lubricating oil 37 in centrifugal pump 41 is pumped up by the centrifugal force generated by the rotation of shaft 38. Then, through viscous pump 42, lubricating oil 37 is supplied to each sliding portion. Then, lubricating oil 37 is released from vertical hole 43 and horizontal hole 44, and dispersed to closed-vessel space 32. At this point, lubricating oil 37 dispersed through release passage K reaches the top part of piston 49 located at notched portion 52. Then, oil reservoir 37a is formed on piston 49 at a top part of oil groove 51 by surface tension.
Lubricating oil 37 in oil reservoir 37a flows to the entire circumference of oil groove 51 so as to improve the seal effect between piston 49 and cylinder 46 for reducing leakage loss.
However, in the conventional structure disclosed in above PTL1, the clearance between piston 19 and cylinder 16 is conic trapezoid. This makes spatial volume of clearance larger than when the clearance between piston 19 and cylinder 16 is cylindrical. As a result, when the refrigerant gas is compressed to high pressure and reaches a predetermined pressure, lubricating oil 7 in the clearance may be easily blown into closed-vessel space 2 by high-pressure refrigerant. Therefore, lubricating oil 7 needs to be sufficiently and reliably fed to the clearance between piston 19 and cylinder 16 in order to reduce the leakage loss of refrigerant.
In the conventional structure disclosed in above PTL2, lubricating oil 37 can be reliably fed near to the top dead point of piston 49 by providing oil groove 51 on the outer circumference of piston 49. However, conic trapezoidal inner diameter of cylinder 46 increases the spatial volume of clearance, compared to that of tubular cylinder 46. To increase the oil amount of lubricating oil 37 to be supplied, a capacity of oil groove 51 needs to be enlarged.
However, in a state that refrigerant gas is at high temperature and high pressure in compression chamber 47 near the top dead point of piston 49, lubricating oil 37 carried through oil groove 51 is carried out to the clearance between piston 49 and cylinder 46. Refrigerant gas then flows into the space of oil groove 51, and the space thus becomes a dead volume. Accordingly, reexpansion loss may increase if the capacity of oil groove 51 is increased.
Taking into account that oil groove 51 becomes a dead volume after the refrigerant gas is carried out, it is necessary to secure sufficient oil amount while suppressing the dead volume.
FIG. 22 is a sectional view of a compressor unit, in which refrigerant can be compressed, disclosed in PTL3.
Cylindrical hole portion 66 includes expanded clearance portion 67 and uniform clearance portion 66. Piston 73 has the uniform outer diameter over the entire length. Expanded clearance portion 67 has inner diameter that increases from Dt to Db (>Dt) from the side of the top dead point toward the bottom dead point of piston 73. Uniform clearance portion 68 has a fixed inner diameter in the axial direction, and is formed only for length L at an area corresponding to an end of piston 73 reaching the top dead point to the side of compression chamber 65.
Blowby, which is leakage of high-temperature and high-pressure refrigerant gas compressed in the compression chamber, scarcely occurs up to a midway of a compression stroke before piston 73 reaches near the top dead point by provision of this expanded clearance portion 67 and uniform clearance portion 68. In addition, the sliding resistance of piston 73 reduces. In a state that the compression stroke further advances, and piston 73 comes close to the top dead point, leakage of refrigerant gas in line with increased gas pressure can be reduced, compared to the case of forming the expanded clearance portion over the entire length.
However, in the compressor unit with the conventional structure disclosed in PTL3, the entire piston 73 remains inside cylindrical hole portion 66 even when piston 73 returns to the bottom dead point. Accordingly, the lubricating oil may not be sufficiently supplied between cylindrical hole portion 66 and piston 73 where lubrication is needed.
In addition, in the compressor unit with the conventional structure disclosed in PTL3, the lubricating oil is pushed out when the clearance becomes narrower as piston 73 comes close to the top dead point. When piston 73 returns to the bottom dead point and the clearance becomes broader, the lubricating oil for sealing the clearance is insufficient. This makes it difficult to suppress blowby. Insufficient lubricating oil also increases the sliding resistance.